Control of a helicopter is effected by varying the pitch of the rotor blades individually as the rotor rotates and by varying the pitch of all of the blades together. This is known respectively as cyclic and collective pitch control. Blade pitch control of a helicopter main rotor is typically achieved by using a swashplate which transfers the motion of non-rotating control members to the rotating members. The swashplate is typically concentrically mounted about a rotor shaft. The swashplate includes two concentrically oriented rings connected by a series of bearings with one ring connected to the airframe (stationary), and the other ring connected to the rotor shaft or hub (rotating). Collective control is achieved by translating the swashplate up and down with respect to the rotor shaft and cyclic control is achieved by tilting the swashplate relative to the rotor shaft. The stationary ring is typically centered about the rotor shaft by a spherical ball joint that allows for tilt of the swashplate, with a standpipe surrounding the rotor shaft allowing translation of the swashplate. Pitch links connect the rotating ring of the swashplate to the pitch or control arms of the rotor blades. The stationary ring of the swashplate is positioned by links or servos which are actuated in response to the pilot's control signals. Thus, when the pilot wishes the swashplate to be raised, lowered or tilted, the pilot directs, through the links or servos, the stationary ring to be raised, lowered or tilted and thus every point on the rotating ring of the swashplate is raised or lowered vertically in a collective or cyclic manner as the rotor rotates. Thus, the swashplate must be capable of moving vertically, i.e., in a direction parallel to the rotor shaft and be able to tilt with respect to the plane of rotation.
Various prior art methods are known for connecting the swashplate to the rotor shaft. For example, one or more pivoted link devices may be used, one end of each device being connected to a ring member and the other end being connectable to the rotor while another one or more pivoted link devices is connected to the stationary ring and to the airframe. These links allow the swashplate to be angularly deflected to allow the ring to tilt or to move vertically with respect to the rotor shaft.
Various problems are encountered with the prior art linkages. For example, since these parts are subject to a substantial amount of movement, rapid wear is typical and these parts are generally high-maintenance items. In addition, such linkages are particularly vulnerable when used in military aircraft where a ballistic impact may cause failure of the swashplate control system.
The rotating ring is connected to the rotor shaft through the pivoted link devices typically referred to as "scissors", with the static ring similarly connected to the airframe. These scissors permit the rings to translate vertically, i.e., move parallel to the rotor shaft and to tilt about a diametral axis of the rings, while maintaining the rings azimuthal position with respect to the rotor. Each of the scissors comprises two links connected together by a hinge which permits the scissor link to flex vertically. Typically, one link end is mounted to a spherical bearing which is mounted to the ring to prevent independent rotation of the ring, yet permits it to tilt and translate vertically.